Sex differentiation, changes in lenth, weight and eye size before and after metamorphosis of european eel (Anguilla anguilla L.) maintained in captivity

Migrating European eel (Anguilla anguilla L.) glass eels were collected annually in the Garonne Basin (France) during 1987–1989 and subsequently raised under freshwater conditions. In 1990 and 1991 sex differentiation, gonadal development, length/weight and eye size were studied in relation to metamorphosis from yellow to silver eel stage. Differentiation from intersexual gonads to testes took place after 2 years and was synchronized with metamorphosis. Differentiated ovaries, however, were already present in 2 + yellow eels indicating that expression of female gonadal sex was not synchronized with metamorphosis. Body length and eye size were used to assess the ontogenetic stage of individual fish. The majority of female yellow eel and male and female silver eel of the experimental fish could be identified using these external morphological characteristics. Intersexual yellow eel could not be identified with this method. Intersexual and female yellow eel reached the silver stage at ages of 2–3 and 3–4 years and at body weight ranges of 78–410 and 309–830 g, respectively, implying that metamorphosis in captivity took place at earlier ages and at heavier body weights than in nature. Metamorphosis with respect to enlargement of eyes up to the size of mature silver eels occurred in the hatchery population without hormonal intervention. The testes of silver eels contained spermatogonia in mitotic arrest and the ovaries had oocytes, which were blocked in late prophase of meiosis. Body length and eye size were not suitable characteristics to assess sexual maturity in hatchery raised silver eel. Males and females continued to eat and grow after they became silvery whereas gonadal development gradually stopped. The relationship between age, length/weight, feed intake and metamorphosis (including integumental colour change, enlargement of eyes, degeneration of the alimentary tract, gonadal growth/maturation) as well as the role of gonadotropins are discussed.